Therapeutic ultrasound in general, and High-Intensity Focussed Ultrasound (HIFU) in particular, is rapidly emerging as a promising tool for non-invasive ablation of cancerous, diseased or fat tissue, and as a means of inducing localized heating for the purpose of hyperthermia-enhanced drug release.
For all of the above applications, a major factor limiting the use of therapeutic ultrasound is the lack of a technique that makes it possible to monitor changes in tissue properties at the ultrasound focus non-invasively.
When ultrasound is used to induce mild hyperthermia, the major effect at the ultrasound focus will be a mild increase in tissue temperature. When ultrasound is used to induce ablation, the primary effect will be a dramatic change in the viscoelastic properties of tissue (in a manner similar to cooking a steak).
Clinically available monitoring techniques include the use of B-mode pulse-echo ultrasound, which is only sensitive to changes in the acoustic impedance, speed of sound and attenuation of tissue; a major limitation of this technique is that it cannot be used during ultrasound exposure, because the signal from the therapy transducer will saturate that from the diagnostic transducer. Performing the procedure inside an MRI scanner is another clinically available alternative: MRI is capable of providing temperature measurements, albeit with poor spatio-temporal resolution, and is primarily prohibitive by virtue of its cost. Importantly for tissue ablation applications, it should be noted that temperature is an indirect indicator of tissue damage, and a more direct indicator is therefore desirable.
Under the right conditions, an ultrasound wave propagating through tissue will excite micron-sized bubbles, a phenomenon known as acoustic cavitation. These bubbles can either be spontaneously nucleated by the ultrasound, by making use of gas dissolved in the surrounding tissue, or injected intravenously, such as by using ultrasound contrast agents.